The present invention relates to precision screws used to make ultra-fine adjustments to the positions and/or angular orientations of optical components.
Adjustable mounting apparatuses are commonly used in applications, such as interferometry and holography, in which precise positioning of optical components is essential. Such optical mounts typically comprise a pair of parallel plates: a base plate, which is rigidly fixed to a supporting base or surface, and a stage plate, on which optical components, such as mirrors, lenses, diffraction gratings, prisms, beam-splitters, light sources, and lasers, are mounted. Typically, the stage plate is coupled to the base plate by one or more compressive means—usually springs—which urge the two plates together. Countering the tensioning force of the springs are multiple kinematic adjustment members, which are typically ultra-fine pitch screws that control the position of the stage plate relative to the base plate.
In conventional optical mounts, there are six points of contact between the adjustment screws and the stage plate so as to fix the six degrees of freedom of movement—three translational and three rotational—between the two plates. The contact end of the adjustment screw often has a hemispherical shape, hence such screws are often referred to as “ball-tipped”. The areas on the stage plate where the adjustment screws make contact are typically detents having a concave hemispherical, conical, or multi-planar configuration. For high-precision optical applications, it is essential that the contact between the adjustment screw and the detents be kinematic, low friction, stable, consistent, and reproducible.
A major impediment to a stable and consistent contact between the ball of the adjustment screw and the stage plate detent is the potential for deformation of the detent contact surfaces under the stress of the ball-tip applied against the resistance of the springs. The potential for deformation of the contact surfaces is exacerbated by the fact that the hemispherical tip of the adjustment screw must rotate within the detent as the screw turns, thus applying radial as well as axial stresses on the contact surfaces.
The prior art in this field has approached this problem by seeking to optimize the contact surfaces of the stage plate. In this vein, the U.S. Pat. No. 5,953,164 of Arnone et al., and the published PCT application of Arnone, 2006/055399, teach the use of three kinematic adjustment screws in conjunction with three different types of contact surfaces. The three contact surfaces provide one, two and three degrees of constraint, respectively, which cooperate to provide the requisite six degrees of constraint. While these designs do tend to reduce axial stress by minimizing the points of contact between the ball-tip of the adjustment screw and the detent surface of the stage plate, the ball-tip will still rotate within the contact detent as the screw turns, thus potentially deforming and/or abrading the contact surfaces over time.
Another approach to the problem taught by the prior art involves interposing a bearing or ball between the threaded end of each of the adjustment screws and the cooperating contact surface. Such a design is disclosed in U.S. Pat. No. 5,694,257 of Arnone et al. Here again, however, the rotational torque of the adjustment screws is applied directly to the ball members, causing them to rotate against the contact surfaces. In order to reduce the potential for deformation/abrasion of the contact surface, the Arnone patent employs sapphire pads inserted into the contact detents. But the use of such sapphire pads has several disadvantages. There is the obvious expense associated with the use of sapphire material. Also, the sapphire pads allow only one point of contact between each of the adjustment screws and the stage plate, thereby providing only one degree of constraint per screw and requiring additional means beyond the three adjustment screws to achieve the requisite six degrees of constraint.
The present invention represents a significant advance over the prior art insofar as it integrates an unconstrained spherical member within a cavity in the proximal end of the kinematic adjustment screw itself. The contact between the spherical member and the distal end of the cavity is spring-loaded, such that the spherical member is not rotationally coupled to the adjustment screw. Consequently, the spherical member will not rotate against the contact surface of the stage plate as the adjustment screw is turned, thereby eliminating the potential for deformation and/or abrasion of the contact surfaces without the need for special protective features in the contact surfaces themselves.